JP2004230306A - Method for improving activity of photocatalyst consisting of visible light responsive metal nitride and metal oxynitride - Google Patents
Method for improving activity of photocatalyst consisting of visible light responsive metal nitride and metal oxynitride Download PDFInfo
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- 239000011941 photocatalyst Substances 0.000 title claims abstract description 34
- 238000000034 method Methods 0.000 title claims abstract description 23
- 230000000694 effects Effects 0.000 title claims abstract description 19
- 229910052751 metal Inorganic materials 0.000 title description 9
- 239000002184 metal Substances 0.000 title description 9
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- 238000010438 heat treatment Methods 0.000 claims abstract description 42
- 229910021529 ammonia Inorganic materials 0.000 claims abstract description 38
- 229910052723 transition metal Inorganic materials 0.000 claims abstract description 25
- -1 transition metal nitride Chemical class 0.000 claims abstract description 14
- 150000003624 transition metals Chemical class 0.000 claims abstract description 14
- 229910052784 alkaline earth metal Inorganic materials 0.000 claims abstract description 10
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- 229910052788 barium Inorganic materials 0.000 claims abstract description 4
- 229910052791 calcium Inorganic materials 0.000 claims abstract description 4
- 229910052712 strontium Inorganic materials 0.000 claims abstract description 4
- 230000001699 photocatalysis Effects 0.000 claims description 29
- 229910052758 niobium Inorganic materials 0.000 claims description 3
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- 239000001257 hydrogen Substances 0.000 description 22
- 229910052739 hydrogen Inorganic materials 0.000 description 22
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- 238000000354 decomposition reaction Methods 0.000 description 12
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 10
- 229910052697 platinum Inorganic materials 0.000 description 10
- 230000001105 regulatory effect Effects 0.000 description 10
- 229910052757 nitrogen Inorganic materials 0.000 description 9
- 239000007864 aqueous solution Substances 0.000 description 7
- 238000006243 chemical reaction Methods 0.000 description 7
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 7
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 6
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- 239000010955 niobium Substances 0.000 description 3
- 238000005121 nitriding Methods 0.000 description 3
- 238000006902 nitrogenation reaction Methods 0.000 description 3
- 239000000243 solution Substances 0.000 description 3
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- VZSRBBMJRBPUNF-UHFFFAOYSA-N 2-(2,3-dihydro-1H-inden-2-ylamino)-N-[3-oxo-3-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)propyl]pyrimidine-5-carboxamide Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)C(=O)NCCC(N1CC2=C(CC1)NN=N2)=O VZSRBBMJRBPUNF-UHFFFAOYSA-N 0.000 description 2
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- YJLUBHOZZTYQIP-UHFFFAOYSA-N 2-[5-[2-(2,3-dihydro-1H-inden-2-ylamino)pyrimidin-5-yl]-1,3,4-oxadiazol-2-yl]-1-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)ethanone Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)C1=NN=C(O1)CC(=O)N1CC2=C(CC1)NN=N2 YJLUBHOZZTYQIP-UHFFFAOYSA-N 0.000 description 1
- CONKBQPVFMXDOV-QHCPKHFHSA-N 6-[(5S)-5-[[4-[2-(2,3-dihydro-1H-inden-2-ylamino)pyrimidin-5-yl]piperazin-1-yl]methyl]-2-oxo-1,3-oxazolidin-3-yl]-3H-1,3-benzoxazol-2-one Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)N1CCN(CC1)C[C@H]1CN(C(O1)=O)C1=CC2=C(NC(O2)=O)C=C1 CONKBQPVFMXDOV-QHCPKHFHSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
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- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
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- 238000006479 redox reaction Methods 0.000 description 1
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- 150000003839 salts Chemical class 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 229910052715 tantalum Inorganic materials 0.000 description 1
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 description 1
- 229910000314 transition metal oxide Inorganic materials 0.000 description 1
Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は、遷移金属ナイトライドおよび遷移金属オキシナイトライドからなる光触媒の光触媒活性を改善する方法に関する。
【０００２】
【従来技術】
光で触媒反応を行う技術としては、光触媒能を有する固体化合物に光を照射し、生成した励起電子やホールで反応物を酸化、あるいは還元して目的物を得る方法が既に知られている。
中でも、水の光分解反応は光エネルギー変換の観点から興味が持たれている。また、水の光分解反応に活性を示す光触媒は、光吸収、電荷分離、表面での酸化還元反応といった機能を備えた高度な光機能材料と見ることができる。
【０００３】
【非特許文献１】
Ｃａｔａｌ．Ｌｅｔｔ．，５８（１９９９）．１５３−１５５、Ｃｈｅｍ．Ｌｅｔｔ．，（１９９９），１２０７
【非特許文献２】
表面，Ｖｏｌ．３６，Ｎｏ．１２（１９９８），６２５−６４５
【特許文献１】
特開２００２−６６３３３、特許請求の範囲
【０００４】
工藤、加藤等は、タンタル酸アルカリ、アルカリ土類等が、水の完全光分解反応に高い活性を示す光触媒であることを多くの先行文献を挙げて説明している〔例えば、前記非特許文献１および２〕。前記非特許文献２においては、水を水素または／および酸素に分解する反応を進めるのに有用な光触媒材料について解説しており、水の還元による水素生成反応、または酸化による酸素生成反応および水の完全光分解反応用光触媒についての多くの示唆をしている。
また、白金、ＮｉＯなどの助触媒を担持した光触媒などについいても言及している。
【０００５】
しかしながら、ここで解説されているものは、非金属としては酸素を含むものが主である。また、多くの固体光触媒は価電子帯と伝導帯の間にある禁制帯の幅、即ち、バンドギャップエネルギ−が、３Ｅｖよりも大きいため、３ｅＶ未満の低いエネルギーの可視光で作動させることができない。一方、バンドギャップエネルギーが小さく、可視光で電子、ホールを生ずることのできる従来の固体光触媒のほとんどは水の光分解反応等の反応条件下で不安定である。例えばＣｄＳ、Ｃｕ−ＺｎＳ等のバンドギャップは２．４ｅＶであるが酸化的な光腐食作用を受けるため、触媒反応が限定されている。
地表に到達する太陽光のほとんどはエネルギーの小さい可視光であり、太陽光で効率的に多様な触媒反応を進行させるためには可視光で作動しかつ安定な光触媒が必要不可欠である。しかしながら上述のように従来の技術で満足できるものは存在しない。
【０００６】
ところで、前記したように地表で利用できる太陽光のほとんどは可視光であるので、可視光で励起電子とホールを生成でき、かつ種々の酸化および還元反応で安定な光触媒を提供することが本発明の課題である。
従来の安定な光触媒のほとんどは金属酸化物、すなわち非金属元素として酸素を含むものである。このようなものでは、伝導帯及び価電子帯のエネルギー的な位置関係は酸素の価電子、Ｏ２ｐ軌道のエネルギーによって大きく支配されるため、バンドギャップエネルギ−が小さく、可視光で光触媒機能を発現させることができなかった。そこで、本発明者らは、価電子のエネルギーが酸素より高い元素を金属と化合させ、それらの価電子軌道を混成させた場合、価電子帯のエネルギー的位置が高くなり、バンドギャップエネルギーを小さくすることができ、かつ、このような化合物として光触媒反応条件下で安定であるものを見出すことができれば、可視光で作動する新しい光触媒を創出できるものと考えた。
【０００７】
そこで、窒素原子の価電子は酸素原子のそれに比べ高いエネルギーをもつため、窒素原子を含有する金属化合物のバンドギャップエネルギーは金属酸化物のそれに比べ小さくすることができ、適切な量の窒素原子と結合した金属及び金属化合物は長波長の可視光の吸収によって励起電子とホールを生成することが可能となり、可視光で作動する光触媒となるであろうとの推測の下で、更に水の光分解等の反応条件下でも安定である化合物を見出すべく鋭意検討し、金属酸化物、金属塩等の前駆体を大気圧程度のアンモニア気流中で加熱し窒化して、少なくとも１つの遷移金属を含むオキシナイトライドからなる化合物が光触媒として機能することを発見している（前記特許文献１）。その化合物の多くはペロブスカイト結晶構造を取っていることにも言及している（Ｋ． Ｄｏｍｅｎ， Ａ． Ｋｕｄｏ ａｎｄ Ｔ． Ｏｈｎｉｓｈｉ，Ｊ．Ｃａｔａｌ．，１９８６，１０２，９２．）。また、それの光触媒反応における安定性の効果は前記結晶構造によるものと推測している。
【０００８】
本発明者らは、前記原理に基づいて開発した光触媒として、ＬａＴａＯＮ２、ＭＴａＯ２Ｎ（Ｍ：アルカリ土類金属）、Ｔａ３Ｎ５、ＴａＯＮのようなタンタル系、ＬａＴｉＯ２Ｎ、ＴｉＮｘＯｙＦｚのようなチタン系、およびＭＮｂＯ２Ｎ（Ｍ：アルカリ土類金属）のようなニオブ系の化合物を提案してきたが、光触媒の活性の改善の余地があった。
【０００９】
【発明が解決しようとする課題】
本発明の課題は、前記窒化光触媒の光触媒活性を改善する方法を提供することである。
本発明者らは、前記光触媒が可視光活性を持つのは窒素化により酸素原子に代えて窒素原子を導入、置換することによるものであるから、窒素の導入、置換され方は、前記光触媒活性の活性の改善に相関するのではとの推測の下に、窒素の導入、置換の条件としてアンモニア圧を取り上げ、この条件と光触媒の活性の改善との関連を検討する中で、前記窒素化光触媒を少なくともアンモニア圧が０．２ＭＰａより大きな条件、好ましくは０．８ＭＰａ以上の、より好ましくは１０ＭＰａ以上のアンモニア圧の存在下で、３００℃以上で、好ましくは４００℃〜１０００℃で加熱処理することにより光触媒の活性が改善されることを見出し前記課題を解決した。
【００１０】
【課題を解決するための手段】
本発明は、少なくとも１つの遷移金属を含む遷移金属ナイトライドおよび遷移金属オキシナイトライドからなる光触媒を０．２ＭＰａ（メガパスカル）を越えるアンモニア存在下、３００℃以上で加熱処理することを特徴とする前記光触媒の光触媒活性の改善方法である。好ましくは、遷移金属がＬａ、Ｔａ、Ｎｂ、Ｔｉ、Ｚｒからなる群から選択される少なくとも１つであることを前記光触媒活性の改善方法であり、より好ましくは、遷移金属ナイトライドおよび遷移金属オキシナイトライドからなる光触媒がＬａＴａＯＮ２、ＭＴａＯ２Ｎ（Ｍ：アルカリ土類金属）、Ｔａ３Ｎ５、ＴａＯＮ、ＬａＴｉＯ２Ｎ、ＴｉＮｘＯｙＦｚ、およびＭＮｂＯ２Ｎ（Ｍ：アルカリ土類金属）から選択されものであることを特徴とする請求項２に記載の光触媒活性の改善方法であり、また前記アルカリ土類金属がＣａ、ＳｒおよびＢａからなる群から選択される少なくとも１つである前記光触媒活性の改善方法である。
【００１１】
また、好ましくは、アンモニア圧を０．８ＭＰａ以上とすることを特徴とする前記各光触媒活性の改善方法であり、より好ましくは、アンモニア圧を１０ＭＰａ以上、加熱温度を４００℃〜１０００℃とすることを前記光触媒活性の改善方法である。
【００１２】
【本発明の実施の態様】
本発明をより詳細に説明する。
Ｉ．一般的に窒化はアンモニア圧が高いほどよく進むが、前記従来の光触媒は大気圧程度のアンモニア圧で合成されるため窒素化が完全でなく、材料中に少量の窒素の欠陥等が存在することが考えられ、このような欠陥は少量でも光触媒の活性を妨げているものと考えられる。
そこで、これらの欠陥をとり除くのに、先行技術が遷移金属酸化物の窒化により光触媒活性の向上することの原理を押し進めて、窒素の導入、置換の改善を、アンモニア圧などとの関連で鋭意検討し、請求項に記載のアンモニア圧と温度による熱処理が光触媒の活性の改善に有効であること見出した。
【００１３】
ＩＩ．具体的には、前記先行技術の遷移金属ナイトライドおよび金属オキシナイトライド光触媒活性を向上させるためには０．２ＭＰａを越えるアンモニアの存在下で加熱処理を行う必要がある。好ましい処理条件は０．８ＭＰａ以上のアンモニア存在下、３００℃以上で加熱することである。加熱温度が３００℃未満の場合はアンモニアの分解反応がほとんど進行せず、十分な窒化ができない。より好ましい処理は前記遷移金属ナイトライドおよび金属オキシナイトライドを１０ＭＰａ以上のアンモニア存在下、４００℃〜１０００℃で加熱することである。４００℃以上ではアンモニアの分解反応が進行し、効率的に材料の窒化を行えるが、１０００℃を越えるとアンモニアの分解によって生じた水素が上記金属ナイトライド、金属オキシナイトライドの金属イオン（Ｔａ５＋、Ｔｉ５＋、Ｎｂ５＋）を還元し、光触媒活性を著しく低下させる。
【００１４】
【実施例】
以下の実施例により本発明を具体的に説明するが、これによって本発明が限定されるものではない。
実施例１
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理後のＴａ３Ｎ５に白金０．５ｗｔ％担持した材料０．２ｇを８０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は３０μｍｏｌｈ−１であった。処理前のＴａ３Ｎ５における水素生成活性は上記と同様の方法で測定した場合５μｍｏｌｈ−１であった。明らかに高圧アンモニア存在下での加熱処理がＴａ３Ｎ５の光触媒活性を向上させている。
【００１５】
実施例２
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を４００℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を０．２ＭＰａに保持した。
上記処理後のＴａ３Ｎ５に白金０．５ｗｔ％担持した材料０．２ｇを８０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は８μｍｏｌｈ−１であり、明らかに高圧アンモニア存在下での加熱処理がＴａ３Ｎ５の光触媒活性を向上させている。
【００１６】
実施例３
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を０．８ＭＰａに保持した。
上記処理後のＴａ３Ｎ５に白金０．５ｗｔ％担持した材料０．２ｇを９０ｖｏｌ％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は１２μｍｏｌｈ−１であり、明らかに高圧アンモニア存在下での加熱処理がＴａ３Ｎ５の光触媒活性を向上させている。
【００１７】
実施例４
試験管状のステンレス製耐圧容器の底部に１ｇのＴａＯＮをいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。上記処理後のＴａＯＮに白金１ｗｔ％担持した材料０．２ｇを１０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は７０μｍｏｌｈ−１であった。処理前のＴａＯＮにおける水素生成活性は上記と同様の方法で測定した場合３０μｍｏｌｈ−１であった。明らかに高圧アンモニア存在下での加熱処理がＴａＯＮの光触媒活性を向上させている。
【００１８】
実施例５
試験管状のステンレス製耐圧容器の底部に１ｇのＣａＴａＯ２Ｎをいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理後のＣａＴａＯ２Ｎに白金１ｗｔ％担持した材料０．２ｇを１０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は４０μｍｏｌｈ−１であった。処理前のＣａＴａＯ２Ｎにおける水素生成活性は上記と同様の方法で測定した場合１８μｍｏｌｈ−１であった。明らかに高圧アンモニア存在下での加熱処理がＣａＴａＯ２Ｎの光触媒活性を向上させている。
【００１９】
実施例６
試験管状のステンレス製耐圧容器の底部に１ｇのＬａＴｉＯ２Ｎをいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５００℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理後のＬａＴｉＯ２Ｎに白金０．５ｗｔ％担持した材料０．２ｇを１０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は５２μｍｏｌｈ−１であった。処理前のＬａＴｉＯ２Ｎにおける水素生成活性は上記と同様の方法で測定した場合２８μｍｏｌｈ−１であった。明らかに高圧アンモニア存在下での加熱処理がＬａＴｉＯ２Ｎの光触媒活性を向上させている。
【００２０】
実施例７
試験管状のステンレス製耐圧容器の底部に１ｇのＣａＮｂＯ２Ｎをいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理後のＣａＮｂＯ２Ｎに白金１ｗｔ％担持した材料０．２ｇを１０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は３０μｍｏｌｈ−１であった。処理前のＣａＮｂＯ２Ｎにおける水素生成活性は上記と同様の方法で測定した場合８μｍｏｌｈ−１であった。明らかに高圧アンモニア存在下での加熱処理がＣａＮｂＯ２Ｎの光触媒活性を向上させている。
【００２１】
比較例１
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を５５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を０．１ＭＰａに保持した。
上記処理後のＴａ３Ｎ５に白金０．５ｗｔ％担持した材料０．２ｇを８０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したときの、水素生成速度は処理前のＴａ３Ｎ５における水素生成活性と同様であった。
【００２２】
比較例２
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を２５０℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理後のＴａ３Ｎ５に白金０．５ｗｔ％担持した材料０．２ｇを８０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し４２０ｎｍ以上の可視光を照射したときの、水素生成速度は処理前のＴａ３Ｎ５における水素生成活性と同様であった。
【００２３】
比較例３
試験管状のステンレス製耐圧容器の底部に１ｇのＴａ３Ｎ５をいれ、高圧ガス導入装置に接続した。ステンレス製耐圧容器、高圧ガス導入装置内を真空ポンプで排気した後、液体アンモニア（９９．９９９％）をステンレス製耐圧容器に導入した。その後、ステンレス製耐圧容器の温度を１２００℃まで昇温し、この温度で２４時間保持してから室温まで冷却した。なお、昇温中の熱膨張の及び処理温度でのアンモニアの分解反応のため上記加熱処理中にステンレス耐圧容器の圧力は常に上昇するが、圧力調整弁を用いて加熱処理中の圧力を４０ＭＰａに保持した。
上記処理によって得られた黒色粉末に白金０．５ｗｔ％担持した材料０．２ｇを８０ｖｏｌ．％メタノール水溶液０．２００ｄｍ３に懸濁し、４２０ｎｍ以上の可視光を照射したとき、水素生成生成は全く観察されなかった。
【００２４】
【発明の効果】
以上のように、本発明で提案したアンモニア圧および温度において、少なくとも１つの遷移金属を含む遷移金属ナイトライドおよび遷移金属オキシナイトライドからなる光触媒を加熱処理することにより、可視光領域の光による光触媒反応における水素発生が著しく向上していることから、太陽光を次世代エネルギーとしての水素に変換する光触媒として一層有望である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for improving the photocatalytic activity of a photocatalyst comprising a transition metal nitride and a transition metal oxynitride.
[0002]
[Prior art]
As a technique for performing a catalytic reaction by light, a method of irradiating a solid compound having photocatalytic ability with light and oxidizing or reducing a reaction product with generated excited electrons or holes to obtain a target product is already known.
Above all, the photolysis reaction of water is of interest from the viewpoint of light energy conversion. In addition, a photocatalyst that is active in the photolysis reaction of water can be regarded as an advanced photofunctional material having functions such as light absorption, charge separation, and redox reaction on the surface.
[0003]
[Non-patent document 1]
Catal. Lett. , 58 (1999). 153-155, Chem. Lett. , (1999), 1207
[Non-patent document 2]
Surface, Vol. 36, no. 12 (1998), 625-645
[Patent Document 1]
JP-A-2002-66333, Claims
Kudo, Kato et al. Have described many prior documents that alkali tantalate, alkaline earth, and the like are photocatalysts exhibiting high activity in the complete photolysis reaction of water. 1 and 2]. Non-patent Document 2 describes a photocatalytic material useful for promoting a reaction of decomposing water into hydrogen and / or oxygen, and describes a hydrogen generation reaction by reduction of water, an oxygen generation reaction by oxidation, and water There are many suggestions for photocatalysts for complete photolysis reactions.
It also mentions a photocatalyst carrying a promoter such as platinum or NiO.
[0005]
However, those described here are mainly non-metals containing oxygen. Also, many solid-state photocatalysts cannot operate with visible light having a low energy of less than 3 eV because the width of the forbidden band between the valence band and the conduction band, that is, the bandgap energy is larger than 3 Ev. . On the other hand, most of conventional solid photocatalysts having a small bandgap energy and capable of generating electrons and holes with visible light are unstable under reaction conditions such as a photolysis reaction of water. For example, the band gap of CdS, Cu—ZnS or the like is 2.4 eV, but is subject to oxidative photocorrosion, so that the catalytic reaction is limited.
Most of the sunlight reaching the surface of the earth is visible light with small energy, and a stable photocatalyst that operates with visible light is indispensable in order to efficiently perform various catalytic reactions with sunlight. However, as described above, none of the conventional techniques is satisfactory.
[0006]
By the way, as described above, most of the sunlight that can be used on the surface of the earth is visible light, and it is therefore an object of the present invention to provide a photocatalyst that can generate excited electrons and holes with visible light and that is stable in various oxidation and reduction reactions. It is an issue of.
Most of the conventional stable photocatalysts are metal oxides, that is, those containing oxygen as a nonmetallic element. In such a device, the energy positional relationship between the conduction band and the valence band is largely governed by the valence electrons of oxygen and the energy of the O2p orbital, so that the band gap energy is small and the photocatalytic function is exhibited by visible light. I couldn't do that. Therefore, the present inventors have found that when an element having higher valence energy than oxygen is combined with a metal and their valence orbitals are mixed, the energy position of the valence band becomes higher and the band gap energy becomes smaller. It was considered that a new photocatalyst that operates with visible light could be created if such a compound could be found and was found to be stable under such photocatalytic reaction conditions as such a compound.
[0007]
Therefore, since the valence electrons of the nitrogen atom have higher energy than that of the oxygen atom, the band gap energy of the metal compound containing the nitrogen atom can be made smaller than that of the metal oxide, and an appropriate amount of the nitrogen atom can be obtained. Assuming that the bound metal and metal compound can generate excited electrons and holes by absorbing visible light of a long wavelength, they will become photocatalysts that operate with visible light, In order to find a compound that is stable even under the reaction conditions described above, a precursor such as a metal oxide or a metal salt is heated and nitrided in a stream of ammonia at about atmospheric pressure to form an oxynitride containing at least one transition metal. It has been discovered that a compound consisting of a ride functions as a photocatalyst (see Patent Document 1). It is also mentioned that many of the compounds have a perovskite crystal structure (K. Domen, A. Kudo and T. Ohnishi, J. Catal., 1986, 102, 92.). Further, it is presumed that the effect of the stability in the photocatalytic reaction is due to the crystal structure.
[0008]
The present inventors, as a photocatalyst which is developed based on the principle, LaTaON 2, MTaO 2 N ( M: alkaline earth metal), Ta 3 N 5, tantalum-based such as TaON, LaTiO 2 N, TiN x O titanium-based, such as y F z, and MNbO 2 N: have been proposed niobium-based compounds such as (M an alkaline earth metal), there is room for improvement of the photocatalytic activity.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for improving the photocatalytic activity of the nitriding photocatalyst.
The present inventors have found that the photocatalyst has visible light activity because nitrogen is introduced and substituted by a nitrogen atom instead of an oxygen atom by nitrogenation. Under the presumption that it correlates with the improvement in the activity of nitrogen, the introduction of nitrogen, ammonia pressure was taken up as a condition for substitution, and while examining the relationship between this condition and the improvement in photocatalytic activity, the nitrogenation photocatalyst was examined. Heat treatment at a temperature of 300 ° C. or more, preferably 400 ° C. to 1000 ° C. in the presence of an ammonia pressure of at least ammonia pressure of 0.2 MPa or more, preferably 0.8 MPa or more, more preferably 10 MPa or more. As a result, the activity of the photocatalyst was improved, and the above-mentioned problem was solved.
[0010]
[Means for Solving the Problems]
The present invention is characterized in that a photocatalyst comprising a transition metal nitride containing at least one transition metal and a transition metal oxynitride is heat-treated at 300 ° C. or more in the presence of ammonia exceeding 0.2 MPa (megapascal). This is a method for improving the photocatalytic activity of the photocatalyst. Preferably, the method for improving photocatalytic activity is that the transition metal is at least one selected from the group consisting of La, Ta, Nb, Ti, and Zr, and more preferably, the transition metal nitride and the transition metal oxy. photocatalyst LaTaON 2 consisting of nitride, MTaO 2 N (M: alkaline earth metal), Ta 3 N 5, TaON , LaTiO 2 N, TiN x O y F z, and MNbO 2 N (M: alkaline earth metal 3. The method for improving photocatalytic activity according to claim 2, wherein the alkaline earth metal is at least one selected from the group consisting of Ca, Sr, and Ba. This is a method for improving the photocatalytic activity.
[0011]
In addition, preferably, the method for improving each photocatalytic activity is characterized in that the ammonia pressure is 0.8 MPa or more, more preferably, the ammonia pressure is 10 MPa or more, and the heating temperature is 400 ° C. to 1000 ° C. Is a method for improving the photocatalytic activity.
[0012]
[Embodiment of the present invention]
The present invention will be described in more detail.
I. In general, nitridation proceeds better as the ammonia pressure is higher. However, since the conventional photocatalyst is synthesized at an ammonia pressure of about atmospheric pressure, the nitrogenation is not complete, and a small amount of nitrogen defects and the like are present in the material. It is considered that such a defect hinders the activity of the photocatalyst even in a small amount.
Therefore, in order to eliminate these defects, the prior art has pushed the principle of improving the photocatalytic activity by nitriding transition metal oxides, and has been diligently studying the improvement of nitrogen introduction and substitution in relation to ammonia pressure and the like. However, it has been found that the heat treatment using the ammonia pressure and temperature described in the claims is effective for improving the activity of the photocatalyst.
[0013]
II. Specifically, in order to improve the photocatalytic activity of the prior art transition metal nitride and metal oxynitride, it is necessary to perform heat treatment in the presence of ammonia exceeding 0.2 MPa. Preferred treatment conditions are heating at 300 ° C. or more in the presence of 0.8 MPa or more of ammonia. If the heating temperature is lower than 300 ° C., the decomposition reaction of ammonia hardly proceeds, and sufficient nitriding cannot be performed. A more preferred treatment is to heat the transition metal nitride and the metal oxynitride at 400 ° C. to 1000 ° C. in the presence of 10 MPa or more of ammonia. At a temperature of 400 ° C. or higher, the decomposition reaction of ammonia proceeds, and the material can be efficiently nitrided. However, at a temperature of over 1000 ° C., hydrogen generated by the decomposition of the ammonia is converted to the metal ion (Ta 5+) of the metal nitride or metal oxynitride. , Ti 5+ , Nb 5+ ) and significantly reduce photocatalytic activity.
[0014]
【Example】
The present invention will be specifically described by the following examples, but the present invention is not limited thereto.
Example 1
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. In addition, the pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the heating and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held.
After the above treatment, 0.2 g of a material having 0.5 wt% of platinum supported on Ta 3 N 5 was subjected to 80 vol. % Methanol aqueous solution was suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, and the hydrogen generation rate was 30 μmolh −1 . The hydrogen generation activity in Ta 3 N 5 before the treatment was 5 μmolh −1 when measured by the same method as described above. Obviously, the heat treatment in the presence of high-pressure ammonia has improved the photocatalytic activity of Ta 3 N 5 .
[0015]
Example 2
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 400 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. The pressure of the stainless steel pressure vessel is constantly increased during the above heat treatment due to the thermal expansion during the temperature increase and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 0.1 using a pressure regulating valve. It was kept at 2 MPa.
After the above treatment, 0.2 g of a material having 0.5 wt% of platinum supported on Ta 3 N 5 was subjected to 80 vol. % Methanol aqueous solution is suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, the hydrogen generation rate is 8 μmolh −1 , and the heat treatment in the presence of high-pressure ammonia clearly shows the photocatalytic activity of Ta 3 N 5 Has been improved.
[0016]
Example 3
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. The pressure of the stainless steel pressure vessel is constantly increased during the above heat treatment due to the thermal expansion during the temperature increase and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 0.1 using a pressure regulating valve. It was kept at 8 MPa.
When 0.2 g of a material in which 0.5 wt% of platinum is supported on Ta 3 N 5 after the above treatment is suspended in 0.200 dm 3 of a 90 vol% methanol aqueous solution and irradiated with visible light of 420 nm or more, the hydrogen generation rate is 12 μmol h −. 1, is clearly heating in the presence of a high pressure ammonia improve the photocatalytic activity of Ta 3 N 5.
[0017]
Example 4
1 g of TaON was placed at the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. In addition, the pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the heating and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held. 0.2 g of 10 wt. % Methanol aqueous solution was suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, and the hydrogen generation rate was 70 μmolh −1 . The hydrogen generation activity in TaON before the treatment was 30 μmolh −1 when measured by the same method as described above. Apparently, the heat treatment in the presence of high-pressure ammonia has improved the photocatalytic activity of TaON.
[0018]
Example 5
1 g of CaTaO 2 N was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. In addition, the pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the heating and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held.
0.2 g of a material in which 1 wt% of platinum is supported on CaTaO 2 N after the above treatment is added in 10 vol. % Methanol aqueous solution was suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, and the hydrogen generation rate was 40 μmolh −1 . The hydrogen generation activity in CaTaO 2 N before the treatment was 18 μmolh −1 when measured by the same method as described above. Apparently, the heat treatment in the presence of high-pressure ammonia has improved the photocatalytic activity of CaTaO 2 N.
[0019]
Example 6
1 g of LaTiO 2 N was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 500 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. In addition, the pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the heating and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held.
0.2 g of a material having 0.5 wt% of platinum supported on LaTiO 2 N after the above-mentioned treatment was added to 10 vol. % Methanol aqueous solution was suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, and the hydrogen generation rate was 52 μmolh −1 . The hydrogen generation activity in LaTiO 2 N before the treatment was 28 μmolh −1 when measured by the same method as described above. Apparently, the heat treatment in the presence of high-pressure ammonia has improved the photocatalytic activity of LaTiO 2 N.
[0020]
Example 7
1 g of CaNbO 2 N was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introducing device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. In addition, the pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the heating and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held.
0.2 g of a material in which 1 wt% of platinum is supported on CaNbO 2 N after the above treatment is added in 10 vol. % Methanol aqueous solution was suspended in 0.200 dm 3 and irradiated with visible light of 420 nm or more, and the hydrogen generation rate was 30 μmolh −1 . The hydrogen generation activity in CaNbO 2 N before the treatment was 8 μmolh −1 when measured by the same method as described above. Obviously, the heat treatment in the presence of high-pressure ammonia has improved the photocatalytic activity of CaNbO 2 N.
[0021]
Comparative Example 1
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introduction device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 550 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. The pressure in the stainless steel pressure vessel is constantly increased during the above heat treatment due to the thermal expansion during the temperature increase and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 0.1 using a pressure regulating valve. It was kept at 1 MPa.
After the above treatment, 0.2 g of a material having 0.5 wt% of platinum supported on Ta 3 N 5 was subjected to 80 vol. When suspended in 0.200 dm 3 of an aqueous methanol solution and irradiated with visible light of 420 nm or more, the hydrogen generation rate was the same as the hydrogen generation activity in Ta 3 N 5 before the treatment.
[0022]
Comparative Example 2
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introduction device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 250 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. Although the pressure of the stainless steel pressure-resistant container constantly rises during the above-mentioned heat treatment due to thermal expansion during the temperature rise, the pressure during the heat treatment was kept at 40 MPa using a pressure regulating valve.
After the above treatment, 0.2 g of a material having 0.5 wt% of platinum supported on Ta 3 N 5 was subjected to 80 vol. The hydrogen production rate when suspended in 0.200 dm 3 of a 0.2% aqueous methanol solution and irradiated with visible light of 420 nm or more was the same as the hydrogen production activity in Ta 3 N 5 before the treatment.
[0023]
Comparative Example 3
1 g of Ta 3 N 5 was placed in the bottom of a test tube-shaped stainless steel pressure vessel, and connected to a high-pressure gas introduction device. After evacuating the inside of the stainless steel pressure vessel and the high-pressure gas introduction device with a vacuum pump, liquid ammonia (99.999%) was introduced into the stainless steel pressure vessel. Thereafter, the temperature of the stainless steel pressure vessel was raised to 1200 ° C., kept at this temperature for 24 hours, and then cooled to room temperature. The pressure of the stainless steel pressure vessel is constantly increased during the heat treatment due to the thermal expansion during the temperature increase and the decomposition reaction of ammonia at the treatment temperature, but the pressure during the heat treatment is reduced to 40 MPa using the pressure regulating valve. Held.
The black powder obtained by the above process was loaded with 0.2 g of a material having 0.5 wt% of platinum supported thereon at 80 vol. When suspended in 0.200 dm 3 of a 0.2% aqueous methanol solution and irradiated with visible light of 420 nm or more, no hydrogen generation was observed.
[0024]
【The invention's effect】
As described above, at the ammonia pressure and the temperature proposed in the present invention, the photocatalyst including the transition metal nitride and the transition metal oxynitride containing at least one transition metal is subjected to the heat treatment, whereby the photocatalyst by the light in the visible light region is obtained. Since the generation of hydrogen in the reaction has been significantly improved, it is more promising as a photocatalyst for converting sunlight into hydrogen as next-generation energy.
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